1. Field of the Invention
This relates to method and apparatus for testing whether a tank completely filled with liquid is leaking and more particularly to leak detection apparatus which will maintain a substantially constant head pressure on the tank during testing to determine if the tank is leaking.
2. Description of the Prior Art
Fuel storage tanks are typically stored underground. If a fuel tank leaks, it poses a serious health and environmental hazard to water supply systems and the like. Sometimes, relatively small leaks can, over a long period of time, cause substantial underground contamination.
It thus become important that method and apparatus be provided for determining the change in volume of fluid in the tank. To accomplish such tests, a standpipe has been heretofore utilized in connection with a tank. One prior art device, such as that disclosed in U.S. Pat. No. 3,580,055, issued to Richard B. White on May 25, 1971, includes a standpipe for establishing a fluid head in communication with the fluid in the tank to measure the change in volume of the fluid in the tank during the test period. This prior art system thus utilized a standpipe to apply additional head pressure to an underground storage tank during testing to determine if the tank leaks. The prior art device of the type disclosed included apparatus for returning or adjusting the height of the fluid head after the test is completed.
One of the problems common to the use of such a standpipe, however, is a diminishing leak rate. As the level in this prior art standpipe changes, either upwardly or downwardly, the level of head pressure also changes. By way of example, if the liquid level in the standpipe went down during the test, so would the amount of the pressure being exerted on the leak. Therefore, as the head pressure would decrease, so would the overall pressure on the inside of the tank and thus the leak rate would also decrease.
The current environmental standard as published by the "Environmental Protection Agency" as N.F.P.A. 329 sets forth the standard for the maximum leakage allowable in fuel tanks. Obviously, if there is a leak in the system and fuel is being emitted from the fuel tank, a decreasing volume in the fuel tank can be measured with various devices. The measurement of change of volume, however, must be tempered with the temperature of the fuel as well as a determination for the level of the surrounding water table. For example, an initial indication of a decreasing volume may falsely indicate a leak if the temperature is decreasing causing the volume to shrink due to temperature change.
The use of a standpipe to provide additional head pressure compensates for a high water table. For example, with a high water table, the pressure outside an underground tank could be equal to or greater than the pressure inside the tank thereby masking a leak. Also, the additional head pressure provided by the standpipes stabilizes vapor pockets that otherwise form at the top of a tank. Vapors can expand and contract with temperature change and unless removed or stabilized by pressure, could mask a leak. The additional head pressure applied to an air pocket by a standpipe will force it to be contained.
Additional head pressure provided by the standpipe is also utilized to stabilize the tank ends. As pressure builds in a tank, the tank ends commonly bulge and could mask a leak. The additional head pressure provided by the standpipe forces the tank ends to deflect to the maximum. As the level of the fluid in a typical standpipe lowers, the pressure on the overall tank is less and therefore, the tank walls will tend to contract because of less head pressure. This contributes to the error in test measurements. Accordingly, it is another object of the present invention to provide constant level leak detection apparatus and method which maintains a constant head pressure on the tank during testing.
It is another object of the present invention to provide a constant level leak detection system which substantially eliminates tank end deflection and declining leak rates due to liquid level changes in the detection system.
It is a further object of the present invention to provide leak test apparatus which will increase the accuracy of testing.
Another object of the present invention is to provide a new and novel leak detector which will eliminate tank end deflection due to changing head pressure.
Another object of the present invention is to provide a leak detector which will eliminate diminishing leak rates. Another object of the present invention is to provide a method and apparatus for leak testing which is more reliable and precise and eliminates diminishing leak rates as a factor for which there must be compensation.
It is another object of the present invention to provide a leak detector which is safer than other prior art detectors which utilize gas under pressure for maintaining a static head pressure.
Another object of the present invention is to provide a leak detector which utilizes liquid and not a gas above the volatile fuel for maintaining the constant head on the fuel.
It is still another object of the present invention to provide constant level leak detection apparatus and method which utilizes a single reservoir which is alternately positioned to supply liquid to or receive liquid from the tank being tested, depending on whether the liquid level in the tank is decreasing or increasing, respectively.
The following prior art patents are cited as being of interest to this construction although they do not disclose the invention disclosed herein:
______________________________________ 4,442,702 Sawada Apr. 17, 1984 4,474,054 Ainlay Oct. 2, 1984 4,571,987 Horner Feb. 25, 1986 4,649,739 Horner Mar. 17, 1987 4,679,425 Bolland Jul. 14, 1987 4,708,015 Sharp Nov. 24, 1987 ______________________________________
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art a the description thereof proceeds.